on specifiers to look for alternative means of providing power, heat and cooling for buildings. As a result, combined heat and power (CHP) has increased in popularity, and there are now more than 1,500 non- domestic CHP installations across the UK. A long-established CIBSE guide on

applying CHP has been substantially updated to reflect the latest trends in energy supply and demand. The structure of the document, AM12: Combined Heat and Power for Buildings, has not altered greatly from the 1999 version, but there has been a complete overhaul and replacement of much of the material. The UK government has an expectation

that there can be a 34% reduction in CO2 emissions by 2035, with decarbonisation of the electricity supply playing a significant part in this. The authors of the updated AM12 are confident that properly-applied CHP is a technology that can reduce emissions as well as being cost-effective to implement in many applications. Comparing gas-fired central power stations with localised CHP (employing current technologies) the revised guide advises that there is a potential to save up to 22% of CO2 emissions, if the electricity and, crucially, the heat can be fully utilised. But it is only through this appropriate application of CHP that significant savings on fuel and environmental emissions may be made. Buildings with a high year-round

34 CIBSE Journal December 2011

demand for heat are ideal; without a heat load, the CHP will switch off and no savings will be made. The guide includes the methodologies

required to assess the viability of CHP, its procurement, installation, commissioning, operation and maintenance. A set of practical applications and ‘lessons learned’ provide insight taken from real-world experiences in the application of CHP.

Small-scale and micro-CHP Since the 1999 edition, there has been significant commercial development of smaller-sized CHP aimed at commercial and residential applications. These systems have been designed specifically to provide longer running hours between servicing and employ more sophisticated control systems. At the larger end of this scale, high-speed

micro-turbines have the benefit of longer maintenance intervals than reciprocating engines, so availability is higher and maintenance costs lower. The guide explains that this benefit is at a cost of an electrical efficiency that is generally lower than the equivalent sized spark-ignition gas engine. Smaller, domestic applications (typically

less than 5 kWe) that avoid the need for the cost of some form of district heating are available. These are seen as potential replacements for the domestic gas boiler, but the heat demands need to be carefully assessed to ensure the units run for significant parts of the year. AM12 notes that these applications are